1. Field of the Invention
The present invention relates to printers in which a platen gap formed between an impact printing member and a platen is varied in size to accommodate different forms thicknesses, and more particularly to printers in which a platen is rotated about an axis to adjust the size of the platen gap.
2. History of the Prior Art
Printers in which a platen gap must be adjusted to accommodate different forms thicknesses are known in the art. An example of such a printer is provided by U.S. Pat. No. 3,941,051 of Barrus et al., PRINTER SYSTEM issued Mar. 2, 1976 and commonly assigned with the present application.
The Barrus et al. patent describes a dot matrix line printer having an elongated hammerbank driven in reciprocating, bi-directional fashion by a cam drive assembly. A ribbon deck mounted within the printer adjacent the hammerbank disposes a length of ink ribbon between a plurality of hammer springs mounted in spaced-apart, parallel fashion along the length of the hammerbank and an adjacent platen. One or more lengths of print paper are disposed between the length of ink ribbon and the platen and are stepped through the print station defined by the space between the hammerbank and the platen by a tractor drive arrangement.
The printer described in the Barrus et al. patent performs printing in dot matrix fashion. As the hammerbank is reciprocated back and forth across the print paper, various ones of the hammer springs along the length of the hammerbank are released or "fired" from the spring-loaded retracted positions in which they are normally held, using associated magnetic hammer actuators. As each hammer spring is fired, an upper free end thereof which mounts a dot printing impact tip thereon flies forward out of the retracted position so that the dot printing impact tip impacts the length of ink ribbon against the print paper to print a dot. The hammer spring then rebounds into the retracted position. Each of the magnetic hammer actuators includes a permanent magnet coupled to the upper free end of an associated one of the hammer springs through a pole piece. The permanent magnet normally holds the hammer spring in the spring-loaded retracted position in readiness for firing. The magnetic hammer actuator also includes a magnetic coil surrounding the pole piece and operative, when momentarily energized, to overcome the effects of the permanent magnet and release the hammer spring to print a dot on the print paper.
In dot matrix impact printers of the type described in the Barrus et al. patent, paper forms having different thicknesses are disposed within the print station where they are supported by the platen. The paper form may comprise a single thickness of print paper or it may comprise a multi-paper form with carbons interposed between adjacent layers of paper. The thickness of a given length of paper may also vary depending on the type and quality of the paper.
Because of the different forms thicknesses, the platen gap within the print station which is defined by the distance between the hammer springs and the platen and which is generally uniform in size along the length of the hammerbank must be variable to accommodate the different forms thicknesses. It has been found that the size of the platen gap is quite important, and even minor variations in the platen gap for a given forms thickness can seriously affect print quality.
In printers of the type described in the Barrus et al. patent, the platen is typically mounted for rotation about an axis, with rotation of the platen being provided by a manually manipulated platen handle coupled to one end of the platen. As the platen handle is manually rotated to rotate the platen, the paper supporting surface of the platen within the print station varies in the distance thereof from the axis of rotation of the platen in eccentric fashion so as to adjust the size of the platen gap. Once the platen gap is adjusted, the friction of the platen mounting mechanism tends to maintain the angular position of the platen and therefor the size of the platen gap defined thereby. To facilitate positioning of the platen handle to achieve the desired platen gap, the platen handle is typically provided with a pointer and is positioned next to a scale. As the platen handle is rotated the pointer thereon resides at different locations along the scale designating the proper platen gap for different forms thicknesses.
Conventional platen gap adjusting arrangements are typically somewhat deficient in their ability to provide precision platen gap setting and maintenance of the setting thereafter. Due to the frictional nature of the positioning typically used, gap size can change or "creep" due to such things as vibration of the printer and paper drag on the platen as the paper is continuously advanced over the platen by the tractor drives. The pointer and scale arrangement used for setting and thereafter resetting the platen gap is not always accurate. Quite often, trial and error are required to obtain an accurate initial setting, which process often must be repeated to reset the gap later. Also, when the printer is shipped, it is usually necessary to place a constraining device in the platen gap to prevent creep of the platen into a position in which the platen can damage the hammerbank.
Accordingly, it would be desirable to provide a mechanism for precisely and accurately selecting the proper platen gap with the gap thereafter being maintained in spite of printer vibrations, paper drag and the like. It would also be desirable to provide a mechanism for precisely and accurately resetting the platen gap to a desired size after the gap has been changed.